1. Field of the Invention
The present invention relates to a power tool having a tool bit that performs a predetermined operation by rotating on its axis and more particularly, to a power tool having a torque limiter that selectively interrupts transmission of torque to the tool bit.
2. Description of the Related Art
Japanese non-examined laid-open patent publication No. 9-57511 discloses a hammer drill having a torque limiter. The known torque limiter has a torque transmitting ball between a driving-side rotating member in the form of a driving gear and a driven-side rotating member in the form of a driven flange which are coaxially dispose The ball is held in a radial holding groove formed in the driven flange and engages with a cam of the driving gear in the circumferential direction so that the torque of the driving gear is transmitted to the driven flange. When the torque acting between the driving gear and the driven flange exceeds a predetermined torque, the ball moves radially inward along the holding groove against the biasing formed of an elastic member (disc spring) via a pressing plate. Thus, the engagement of the ball with the cam of the gear is released and the torque transmission is interrupted.
It is essential for a torque limiter to constantly operate at a predetermined torque or to be configured to interrupt the torque transmission when the torque acting upon the driven flange exceeds a predetermined set value. The known torque limiter is configured such that the biasing force of the elastic member constantly acts upon the ball via the pressing plate. Therefore, the ball is not allowed to roll and the ball comes in point contact with the driving gear, the driven flange and the pressing plate. Thus, local wear is caused and in this connection, further improvement is required in durability.